1. Field of the Invention
The present invention relates to an electronic device for providing reference voltages, and more particularly to a reference voltage generator that can mitigate kickback noise interference to provide a steady reference voltage for an analog-to-digital converter.
2. Description of the Prior Art
An electronic device commonly requires a reference voltage generating circuit for providing reference voltage levels to internal circuits. An ideal reference voltage generating circuit is a voltage generator that can generate a fixed voltage that does not vary with temperature, supply power variation, or kickback noise. An analog-to-digital converter (ADC) requires the reference voltage generating circuit to provide an input voltage range and converting levels. For example, a 10-bit, 20 MHz pipeline ADC generally operates with three reference voltages: 1.525V, 1.4V and 1.375V. If the reference voltage generating circuit generates reference voltages imprecisely or unstably, the ADC may convert input signals distortedly. Hence, how to make cooperation between the reference voltage generating circuit and the ADC perfect and stable over bandwidth, noise, and operation-rate dominates converting performance of the ADC.
Please refer to FIG. 1, which is a schematic diagram of a reference voltage generator 100 according to the prior art. The reference voltage generator 100 includes amplifiers 110 and 120 with unity gain and capacitors 112 and 122, and converts the reference voltages REF1 and REF2 for an ADC unit 130 through the amplifiers 110 and 120. For absorption of the high-frequency noise component, the capacitors 112 and 122 are adopted to couple to the outputs of the amplifiers 110 and 120 so that the output signals remain steady at high frequencies. However, as the operation rate of the ADC unit 130 increases, the reference voltage generator 100 requires the capacitors 112 and 122 have larger capacitance. The capacitors 112 and 122 become large, resulting in space being occupied in the reference voltage generator 100.
To solve this, US Patent Publication no. 2006/0187108, entitled ‘Reference Voltage Driving Circuit and Pipeline Analog to Digital Converter Including Same’, discloses a reference voltage driver including two source followers for converting reference voltages REFT and REFC to output appropriate voltages to an ADC at the back end. As can be seen from FIG. 3 of 2006/0187108, the transistor MPT2 controls a current of the transistor MPT1 via a bias voltage PBIAS, and outputs a voltage RTOP_MDAC from the drain after the reference voltage REFT is converted by the transistor MPT1. Thus, the reference voltage driver can operate at high frequency to cooperate with a high-speed ADC. However, the reference voltage REFT may be disturbed by variation of the voltage RTOP_MDAC, causing operation errors of the ADC. Hence, the reference voltage driver cannot significantly resist the feedback signal disturbance.
In addition, US Patent Publication no. 2006/0202876, entitled ‘Reference Voltage Supplying Circuit and Analog-to-Digital Converter Equipped Therewith’, discloses a reference voltage supplying circuit. According to 2006/0202876, the operational amplifier (OP) generates a reference voltage level, and two voltage generators then adjust the reference voltage level to output a maximum reference voltage and a minimum reference voltage to the ADC circuit. However, in the reference voltage supplying circuit, the OP cannot perfectly cooperate with the ADC circuit due to the operating bandwidth limitation. Besides, as the output signals RTOP and RBOT are disturbed by feedback noise from the ADC circuit, the reference voltage level outputted from the OP is affected as well. In this situation, the reference voltage supplying circuit demands an OP having wide operating bandwidth and great stability to solve the above-mentioned problems.
As mentioned above, the reference voltage circuit of the prior art cannot significantly resist signal disturbance from the outputs to the inputs, and has insufficient operating bandwidth or a lack of stability.